Metal-ceramic substrates are preferably used in the field of semiconductor power modules. In doing so, a ceramic plate or ceramic layer, such as an aluminum-oxide ceramic, is provided on at least one of its surface sides, such as the top and/or bottom side, with a metallization, wherein a circuitry structure, such as conductive paths, contact surfaces, and/or connective surfaces, produced, for example, by etching processes, is introduced into at least one metallized side. Such ceramic substrates with metallization are used, for example, as circuit carriers for electronic power modules, in which they ensure the thermal and mechanical connection, as well as the electrical insulation.
The application of a metallization to a ceramic substrate can take place using a method as described in DE 23 19 854 A, for example. In the process, a metal part, such as a copper plate or a copper foil, is provided on the surface sides with a coat made of a chemical compound of the metal and a reactive gas—in particular, oxygen. This coat forms a eutectic with a thin layer of the adjacent metal, the eutectic having a melting temperature below the melting temperature of the metal. The metal part is then placed onto the ceramic substrate and heated together with the ceramic to a temperature above the melting point of the eutectic and below the melting temperature of the metal. Essentially, only the eutectic intermediate layer is thereby melted. After cooling, the metal part and the ceramic substrate are then joined to each other. When using copper or a copper alloy as the metal, this method is also called DCB bonding or DCB process (DCB: Direct Copper Bonding), but the method can also be performed using other metals. The DCB process comprises, for example, the following process steps:                Oxidizing of a copper foil, such that an even copper oxide layer is formed;        Placing the copper foil onto the ceramic layer;        Heating of the composite to a process temperature between approximately 1025 and 1083° C., e.g., to approximately 1071° C.; and        Cooling to room temperature.        
The material composite obtained thereby, i.e., the metal-ceramic substrate, can then be processed further in the manner desired.
Another known method for producing a thick metallization on a ceramic substrate is the so-called active brazing process (AMB: Active Metal Brazing) as used, for example, in DE 22 13 115 A or EP 153 618 A2. In this process, a joint between a metal foil and a ceramic substrate is produced at a temperature between approximately 800 and 1200° C. using a hard solder that also contains an active metal in addition to a main component, such as copper, silver, and/or gold. This active metal, such as at least one element of the group Hf, Ti, Zr, Nb, or Ce, produces a joint between the solder and the ceramic by chemical reaction, while the joint between the solder and the metal is formed by a metallic hard-solder joint.
In both processes, high temperatures are used, whereby the metallization already exerts forces on the underlying ceramic substrate during cooling to room temperature due to the different thermal expansion coefficients. In addition, such a metal-ceramic substrate is subject to thermal fluctuations when used as a substrate for electronic components or assemblies due to the resultant power loss, whereby stress forces on the ceramic layer can develop in the region of the edge of the metallization, which stress forces can result in cracking in the ceramic layer and thus to a destruction of the metal-ceramic substrate or the electronic assembly. In order to avoid such temperature-related stresses, a method for increasing the thermo-mechanical resistance of a metal-ceramic substrate is, for example, known from DE 10 2010 024 520 A1, in which method the edges that exist between the metallization and the ceramic layer are covered by applying an electrically insulating filler material after applying and structuring the metallization. The filler material can, for example, be a temperature-resistant polymer material or a material made of glass or ceramics.
A method for producing a metal-ceramic substrate is also known from DE 10 2013 013 842 B4, in which method cracks existing at the edge region of the metallization between the ceramic and the metal and/or extending into the ceramic are filled or grouted with a curable sealing material, wherein, after the filling or grouting of the sealing material, edge regions of the metallization are covered by the sealing material to a height of at most 50% of the thickness of the metallization.
A task of the metal-ceramic substrate, after the introduction mentioned above of circuitry structures into the metallization, is the separation of different electrical potentials on the same side of the substrate by trench-shaped intermediate spaces in the metallization. e.g., using conductive paths, contact surfaces, and/or connective surfaces spaced apart from each other by the intermediate spaces. These intermediate spaces, herein also called etched trenches, are generally relatively small due to the limited space available or the high packing density on the metal-ceramic substrate, which, consequently, can result in a relatively high electric field strength in these trenches between adjacent metallization regions of differing electrical potentials. Under unfavorable environmental conditions during the use of the metal-ceramic substrate, e.g., in case of hydrogen sulfide (H2S) residues in the air and moisture, electromigration (=dendrite growth) can start at these points of high electric field strength at the metallization and/or its metallic joint to the ceramic layer (active solder). Ultimately, if the electromigration progresses between the metallization regions involved, a short circuit can form between them, which finally results in the destruction of the metal-ceramic substrate or the electric or electronic components—in particular, semiconductor components—carried by them.